Since cross-linked rubbers obtained by vulcanization with sulfur include polysulfide bonds, the heat resistance and vulcanization reversion thereof are poor. To improve these problems of heat resistance and vulcanization reversion, it is known that vulcanization agents such as a tetrasulfide polymer or cyclic polysulfide are effective (Sho Yamazaki, et al.: The Society of Rubber Industry, Japan, 1981 Research Presentation Conference Abstracts, P. 532-17 and Japanese Unexamined Patent Publication (Kokai) No. 10-120788). In particular, a cyclic polysulfide is preferable in terms of cross-linking efficiency, but the production methods of cyclic polysulfides reported until now involve problems such as long production processes, use of expensive materials, etc., and therefore, lack practicality (i.e., Japanese Unexamined Patent Publication (Kokai) No. 58-122944 and Japanese Unexamined Patent Publication (Kokai) No. 2002-293783).
In recent years, pneumatic tires have been improved in various ways. Among these improvements, to raise the heat aging resistance, Japanese Unexamined Patent Publication (Kokai) No. 6-57040 proposes a method of EV cross-linking (i.e., blending in a large amount of a vulcanization accelerator to reduce the ratio of polysulfide bonds), but this had the problem that dynamic fatigue resistance is inferior. Therefore, a method of resolving this tradeoff between the heat aging resistance and the dynamic fatigue resistance is described in Japanese Unexamined Patent Publication (Kokai) No. 2002-293783, but the situation is still insufficient.
Further, as a tire tread rubber for a pneumatic tire, since the improvement of the abrasion resistance or grip is necessary, a rubber composition having a large tensile strength or elongation at break had been sought. On the other hand, tire tread rubber easily degrades. Along with aging, the tread hardens and the grip is decreased. Not only that, in some cases, there was even the danger of the tread peeling off etc. To improve the sustainability of the grip of high performance tires, for example, studies have been conducted on the use of vulcanization agents or vulcanization accelerators, but in particular in rubber compositions having large amounts of filler, it is not possible to satisfy both a grip and its sustainability at satisfactory levels (see Japanese Unexamined Patent Publication (Kokai) No. 2001-348461 and Japanese Unexamined Patent Publication (Kokai) No. 10-151906).
For undertread rubber, to improve the high speed durability, a rubber composition providing a high tensile strength and elongation at break had been sought. On the other hand, a high hardness undertread for improving the steering stability and a low tan δ undertread for improving the fuel efficiency have also been sought. These physical properties were in a tradeoff. From this viewpoint, a rubber composition providing a high hardness and high strength and elongation and not causing a rise in tan δ has been sought.
Further, among the various improvements made to pneumatic tires in recent years, for bead filler rubber, to improve the fatigue resistance, a rubber composition providing a high tensile strength and elongation at break has been sought (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 2002-105249). On the other hand, a high hardness undertread for improving the steering stability and a low tan δ undertread for improving the fuel efficiency have also been sought (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 5-51487). These physical properties were in a tradeoff. From this viewpoint, a rubber composition providing a high hardness and high strength and elongation and not causing the increase in tan δ has been sought.
There is a need for a pneumatic tire which has an emergency running capability enabling it to be run on for a certain distance, even when punctured, bursting, etc. during use on an automobile etc. and rapidly decreasing internal pressure, that is, a run flat tire. To satisfy this need, various proposals have been made. As such a proposal, for example, Japanese Unexamined Patent Publication (Kokai) No. 10-297226 and Japanese National Publication (Tokuhyo) No. 2001-519279 disclose the technology of fitting a run flat support (core ring) over the rims at the inside cavity of a pneumatic tire and using the same to support a punctured or otherwise damaged pneumatic tire so as to enable run flat operation. The rubber used for the side reinforcement layers of such run flat tires is required to feature low heat buildup and high hardness, and therefore large amounts of polybutadiene rubber (BR) have been blended or high cross-linking density formulations have been used. However, the side reinforcement rubber is required to feature strong heat resistance and fatigue resistance, and therefore, if trying to raise the heat resistance by EV cross-linking (that is, blending of a larger amount of vulcanization accelerator than the sulfur to increase the ratio of mono and disulfide bonds), the flex fatigue strength becomes inferior, while conversely if using ordinary cross-linking (that is, increasing the amount of sulfur over the accelerator and increasing the ratio of polysulfides), the flex fatigue strength is improved, but the heat resistance or aging properties become poor, that is to say, there was a problem of antinomy.
Further, side reinforcement rubber formulations contain large amounts of polybutadiene rubber, and therefore even new products (before aging) are low in elongation at break, and therefore there was a limit to the run flat durability after long use (after aging). Further, since run flat tires have thick side parts, at the time of vulcanization, heat conduction inside the tire is difficult. In the case of high temperature vulcanization, a large difference results in the physical properties near the surface and inside, while there was the problem of poor productivity at a low temperature vulcanization.
A run flat core ring 1, as shown in FIG. 1, is arranged in the inner cavity 3 of a pneumatic tire 2, is composed of a ring-shaped metal shell 4 and an elastic ring 5, and is supported at the rim 6. The elastic ring of a run flat core ring is required to have a low heat buildup and high hardness, but, if increased in hardness (the increase in cross-linking density), while maintaining the low heat buildup, the elongation at break become small, and therefore, there was a problem of a limit to the durability during run flat operation. Further, the elastic ring of the core ring 1 is in constant contact with the rim, so the heat generated from the brake at the time of normal operation is transmitted to the rims which are therefore exposed to a considerably high temperature over a long time. Therefore, if the heat aging resistance of the rubber is poor, there has been the problem of the desired performance unable to be exhibited in the run flat state.
Further, the inner liners of pneumatic tires generally use butyl rubbers such as butyl rubber or halogen butyl rubber (e.g., see Japanese Unexamined Patent Publication (Kokai) No. 10-87884), but butyl rubber is poor in reinforcability with carbon black, etc., and therefore, compositions of butyl rubber are inferior in mechanical properties and are limited in applications of use.
Further, belt coat compounds of pneumatic tires are required to provide high rigidity, but if trying to increase the rigidity by increasing the amount of carbon black or increasing the amount of the sulfur serving as the vulcanization agent or vulcanization accelerator, the elongation is decreased and the fatigue resistance is decreased. As a result, separation occurs at the ends of the belt and problems occur in the tire, and therefore, securing of high rigidity and elongation is necessary (Japanese Unexamined Patent Publication (Kokai) No. 2001-226528). Further, to impart a high bonding, blending of a large amount of sulfur is proposed (Japanese Unexamined Patent Publication (Kokai) No. 2000-233603), but this induces deterioration of the heat aging resistance. To increase the heat aging resistance, an antioxidant may be increased, but this has the problem of being liable to impair bonding with wire (metal).